1. Field of the Invention
The invention relates to a process for continuously casting a thin strip, in particular a steel strip, preferably having a thickness of less than 10 mm, in a two-roll process, wherein metal melt is cast into a casting gap formed by two casting rolls in the thickness of the strip to be cast while forming a melt bath and the surfaces of the casting rolls above the melt bath are swept with an inert gas or an inert gas mixture as a function of the condition of the surfaces of the casting rolls, as well as to an arrangement for carrying out the process.
2. Description of the Prior Art
When casting a thin strip in a two-roll process, the cross section of the strip is determined by a section of the casting rolls in a hot state. It is essential that the hot section exactly corresponds to the desired strip cross section, since the strip section can no longer be changed after the casting process, i.e. not even by means of a rolling process. The hot section of the casting rolls deviates considerably from the cold section due to the periodically occurring very high thermal loads exerted on the surfaces of the casting rolls. Thermal cambering will be caused, which, however, may be compensated for at least partially by concave rough-grinding of the casting rolls.
The thermal load exerted on the casting rolls in the casting process is, however, influenced by a plurality of parameters. In addition, a strip caster should encompass a wide operating range. Some examples of parameters and operating ranges include: a casting speed range between 0.2 and 2.5 m/s; a strip thickness range between 1 and 10 mm; different rolling forces occurring on the casting rolls; different temperatures of the metal melt to be cast; and different melt qualities such as, e.g., different steel grades, etc. Because of the variation in operating parameters and ranges, sufficient pre-profiling of the casting rolls by rough-grinding is not feasible. Rather, it is necessary to effect an on-line adjustment of the casting roll surfaces for adaptation to different operating points.
Such an on-line adjustment is known, for instance, from AU-A-50 340/96. There, the surfaces of the casting rolls are observed by sensors coupled to a computer. The computer controls a gas feed to the casting rolls, wherein two different gases, i.e. nitrogen and argon, are fed to the casting rolls, and hence to the melt bath in different partial amounts depending on the condition the surfaces of the casting rolls, in order to influence the heat transfer just above the bath level of the melt bath. The mixed gas thus formed is fed to the surfaces of the casting rolls in a manner distributed over the total longitudinal extent of the same. This is to avoid thermal cambering of the casting rolls and to safeguard a uniform thickness of the strip alternative produced. As an alternative, another suggestion is to measure the thickness of the strip distributed over the width of the strip so as to be able to detect deviations from a rectangular cross section of the strip and compensate for the same by appropriate mixing ratios of the gases fed to the casting roll surfaces. As already mentioned, the heat transfer between the casting rolls and metal melt may be decisively influenced by different gas compositions, thus bringing about changes in the geometries of the casting rolls.
Internal research work in the field of two-roll casting has revealed that a satisfactory product cannot be obtained despite the above-described measures. A uniform surface roughness over the total surface casting rolls is not maintained due to thermal deformation of the casting rolls and due to a slightly uneven solidification of the metal melt on the surface of the casting rolls despite the supply of specifically adjusted gas mixtures. Circumferentially oriented smooth sites not extending over the total longitudinal extent of the casting rolls are also observed. Thus, brighter, smoother sites are, for instance, formed on the circumference of the casting rolls. Since such smooth sites, due to their reduced roughness, cause a more rapid solidification of the metal melt and hence a better contact within the casting gap, the so-called xe2x80x9ckissing pointxe2x80x9d, which, in turn, induces higher local specific rolling forces, the smoothness of the casting rolls in these areas which are already smoother is intensified. This causes a building-up process and hence an ever increasing deterioration of the strip quality, which cannot be obviated by the above-described measures, i.e. a change in the mixing ratio of the gas fed near the bath level.
The present invention seeks to overcome the drawbacks, disadvantages and difficulties in the prior art. A further object of the present invention is to provide a process, as well as an arrangement for carrying out the process, for the production of a strip having an ideal cross section even with strongly varying operating states. The occurrence of thermal deformations of the casting rolls due to local smooth sites is to be avoided, in particular.
In accordance with the invention, the above objects are achieved by a gas sweeping the surfaces of the casting rolls over the longitudinal extent of the casting rolls in a locally different manner.
A preferred embodiment calls for the surfaces of the casting rolls to be observed over their longitudinal extent with respect to locally different conditions. Gas sweeping of the surfaces of the casting rolls is carried out as a function of local observation.
Preferably, locally different gas sweeping is carried out with locally different gas compositions.
Locally different gas sweeping may, however, also be carried out with locally different gas amounts and/or with locally different gas pressures.
Preferably, locally different surface roughness conditions of the casting rolls are observed.
According to another embodiment, locally different surface reflection property conditions of the casting rolls are observed.
It is, however, also possible to observe locally different discolorations of the surfaces of the casting rolls.
A simple realization of the process is feasible if the surfaces of the casting rolls in the direction of their longitudinal extent are divided into consecutively arranged zones. Each zone is observed with respect to the condition of the surfaces, and locally different gas sweeps in zones, i.e. by gas sweeping that is uniform and constant within each zone. At least three adjacently located zones and up to 40 adjacently located zones are preferably formed.
In a preferred embodiment of the present invention the observation of the surfaces of the casting rolls is carried out by receiving electromagnetic waves emitted and/or reflected from the roll surfaces. In particular, the received waves are in the range of visible light and/or in the range of heat radiation.
According to another embodiment of the invention, the condition of the casting rolls is determined indirectly by observing the cast strip over a width of the strip when emerging from the casting gap. At least one surface of the strip is observed over the strip width immediately after the strip emerges from the casting gap. Electromagnetic waves emitted and/or reflected from the surface of the strip, in particular in the range of visible light and/or in the range of heat radiation, are received and measured.
Preferably, gas sweeping is carried out at a pressure on the gas outlet openings of at least 1.05 to a maximum of 2 bar and, preferably, at least 1.5 bar, wherein gas sweeping is expediently carried out at a gas outlet speed at the gas outlet openings of at least 0.2 m/s and, preferably, at least 1.5 m/s.
An arrangement for continuously casting a thin strip by applying a process according to the present invention is also contemplated. The arrangement includes a continuous casting mold formed by two casting rolls defining a casting gap, wherein the width of the casting gap corresponds to the thickness of the strip to be cast. A melt bath receptacle covered by a lid is formed between the casting rolls above the casting gap. A gas feeding device is provided for feeding an inert gas to the casting rolls and has at least one gas outlet opening just above the melt bath between the casting rolls. A device for observing the surfaces of the casting rolls and a unit for influencing or controlling the gas feed to the casting rolls as a function of the condition of the casting roll surfaces is also provided. Several gas feeding devices are provided, wherein each gas feeding device is associated with a partial surface area of a casting roll. Each partial surface area can be fed with gas by means of the associated gas feeding device as a function of an observed value related to a condition of each partial surface area. The control unit can control the gas feeds based on the observations of the casting rolls.
Preferably, each gas feeding device comprises several closely adjacent gas outlet openings.
A preferred embodiment provides gas feeding devices that are connected to two or more gas reservoirs each containing a different gas. The gas feeding devices are supplied via gas ducts equipped with throttle or shut-off members. The gas ducts of each gas feeding device open into a mixing device, preferably a mixing chamber, associated with the gas feeding device. At least one gas feeding duct leads from the mixing device to the gas outlet opening(s) associated with the gas feeding device.
Expediently, the devices for observing the surfaces of the casting rolls are formed by sensors directed towards the surfaces of the casting rolls.
For a particularly thorough observation of the surfaces of the casting rolls, a profile sensor is provided as the sensor for each of the casting rolls. The profile sensor provides an integral observation of the surfaces of the casting rolls over their longitudinal extent, preferably over their total longitudinal extent.
It is also possible to observe the surfaces of the casting rolls, indirectly, i.e. via the cast strip. The devices for observing the surfaces of the casting rolls are formed by sensors directed towards at least one of the surfaces of the cast strip.
According to another preferred embodiment, two or more, preferably at least three, devices for observing the surfaces of the casting rolls are distributed over the longitudinal extent of the casting rolls. Each of the observing devices are separately coupled with a respective gas feeding device via a control unit.
Preferably, the gas outlet openings are oriented in a circumferential direction with axes of the gas outlet openings directed towards the surfaces of the casting rolls. The axes form an angle with a perpendicular to the casting roll surfaces that are within a range of between about +60xc2x0 and xe2x88x9260xc2x0 and, preferably, between about +20xc2x0 and xe2x88x9230xc2x0.
A preferred embodiment is characterized in that the surfaces of the casting rolls have a roughness of more than 4 xcexcm and, preferably, more than 8 xcexcm.
According to a further preferred embodiment, the surfaces of the casting rolls are provided with dimples whose depths are between 10 and 100 xcexcm and whose diameters are between 0.2 and 1.0 mm. The dimples advantageously contact one another, preferably 5 to 20% of the dimples.
Good gas sweeping is ensured if more than 20% of the dimples contact one another.